We arrive from London with BA into Chennai and transit to Colombo.
We only have hand luggage.
How long do we need for the connection and do we need a transit visa?
Immigration could take around 30 minutes. Your flights are both international and should be in the same terminal. The gate will close around 45 minutes before departure.
From this website:
If you meet all of the following conditions, you can transit through India without visa. The conditions are:
Layover time is less than 24 hours as determined by scheduled arrival and departure times;
With confirmed plane tickets to a third country;
Staying in the specific Transit Area (means no clearing immigration, no re-checking your luggage).